This invention relates to medical devices adapted to be implanted in a bodily lumen. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of an implantable medical device. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of an anatomical lumen such as, for instance, a blood vessel, urinary tract or bile duct. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a lumen. In such treatments, stents reinforce the vessel and serve to reduce the incidence of restenosis following angioplasty. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated as by balloon angioplasty or valvuloplasty.
Stents have been made of many materials including metals and polymers. Polymer materials include both biostable and biodegradable polymers. The cylindrical structure of stents is typically composed of scaffolding that includes a pattern of interconnecting structural elements. The scaffolding can be formed from wires, tubes, or planar sheets of material rolled into a cylindrical shape. A stent may be medicated by coating the surface the scaffolding with a polymeric carrier that contains an active agent or drug. Generally, the pattern of the scaffolding allows the stent to be radially expandable and to be longitudinally flexible.
Longitudinal flexibility facilitates delivery of the stent and lateral rigidity is needed to hold open a bodily lumen. The pattern should be designed to maintain the longitudinal flexibility and rigidity required of the stent.
A number of techniques have been suggested for the fabrication of stents from tubes and planar films or sheets. One such technique involves laser cutting or etching a pattern into a material. Laser cutting may be performed on a planar sheet of a material which is then rolled into a tube. Alternatively, a desired pattern may be etched directly onto a tube. Other techniques involve cutting a desired pattern into a sheet or a tube via chemical etching or electrical discharge machining. Laser cutting of stents has been described in a number of publications including U.S. Pat. No. 5,780,807 to Saunders, U.S. Pat. No. 5,922,005 to Richter, and U.S. Pat. No. 5,906,759 to Richter.
A stent must be delivered and deployed. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region requiring treatment. “Deployment” refers to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent at one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter to the desired treatment location, expanding the stent at the treatment location and removing the catheter. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. Once delivered to the treatment location, the stent is expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn allowing the stent to self-expand.
Some treatments with implantable medical devices require the presence of the device only for a limited period of time. Once treatment is complete, which may include structural tissue support and/or drug delivery, it may be desirable for the stent to be removed from the treatment location. One way of removing a stent is by fabricating at least part of the device from materials that erode or disintegrate when exposed to conditions within the body. Thus, erodible portions of the device can disappear or substantially disappear from the implant region after the treatment regimen is completed. After the process of disintegration is completed, desirably no portion of the device will remain although in some cases negligible traces of residue may be observed.
The terms degrade, absorb, resorb, erode and the normal English language versions of these words are used interchangeably and refer to materials that are capable of being substantially completely, preferably completely, desintegrated when exposed to bodily conditions and thereafter being resorbed, absorbed, and/or eliminated by the body.
A potential shortcoming of implantable medical devices made from polymer material compared to metal stents is that polymer stents typically have less circumferential strength and rigidity. Inadequate circumferential strength may result in relatively high recoil of such polymeric devices after implantation into vessels. Furthermore, struts of polymer devices can crack during crimping, especially if the polymer is brittle. Therefore, methods of manufacturing polymer devices that improve circumferential strength and rigidity are desirable.